1) Field of the Invention
The present invention relates to an electrophotographic recording apparatus such as a copying machine, a laser printer or the like, and in particular, relates to an improvement of such an electrophotographic recording apparatus in which a one-component developer is used for recording an image.
2) Description of the Related Art
In an electrophotographic recording apparatus, the following processes are typically carried out:
a) a uniform distribution of electrical charges is produced on a surface of an electrostatic latent image carrying body; PA1 b) an electrostatic latent image is formed on a charged area of the body surface by an optical writing means such as a laser beam scanner, an optical projector or the like; PA1 c) the latent image is developed as a visible image with a developer or toner, which is electrically charged to be electrostatically adhered to the latent image zone; PA1 d) the developed toner image is elect ostatically transferred from the body to a sheet of paper; and PA1 e) the transferred toner image is fixed on the sheet of paper by a toner image fixing means such as a heat roller.
Typically, the electrostatic latent image carrying body may be an electrophotographic photoreceptor, usually formed as a drum and called a photosensitive drum, having a cylindrical conductive substrate and a photoconductive insulating film bonded to a cylindrical surface thereof. In general, the charged area on the drum is produced by an electric discharger such as a corona discharger, and this type of discharger is also used for the transfer of the developed toner image from the drum to the paper.
As one type of developer, a two-component developer, which is well known, is composed of a toner component (colored fine synthetic resin particles) and a magnetic component (magnetic fine carriers). Note, typically the toner particles have an average diameter of about 10 .mu.m, and the magnetic carriers have a diameter ten times larger than the average diameter of the toner particles. Usually, a developing device using this type developer includes a vessel for holding the two-component developer, wherein the developer is agitated by an agitator provided therein. This agitation causes the toner particles and the magnetic carriers to be subjected to triboelectrification, whereby the toner particles are electrostatically adhered to each of the magnetic carriers. The developing device also includes a magnetic roller provided within the vessel as a developing roller in such a manner that a portion of the magnetic roller is exposed therefrom and faces the surface of the photosensitive drum. The magnetic carriers with the toner particles are magnetically adhered to the surface of the magnetic roller to form a magnetic brush therearound, and by rotating the magnetic roller carrying the magnetic brush, the toner particles are brought to the surface of the drum for the development of the electrostatic latent image formed thereon.
In this developing process, a quality of the developed toner image, and therefore the recorded toner image, greatly depends upon an amount of electric charges of the toner, and the amount of electric charges is governed by environmental factors, especially, a temperature and air moisture content. In general, under a low temperature and low air moisture content, the electric charges of the toner become larger, whereas under a high temperature and high air moisture content, the amount of charges of the toner become smaller. When the toner is excessively charged, a density of the toner image is lowered to thereby cause a deterioration of the recorded toner image. On the contrary, as the charges of the toner become smaller, the density of the toner image becomes higher, but an electrophotographic fog appears as a stain on the sheet or paper when the charges of the toner are too small.
A one-component developer is also known, which is composed of only a toner component (colored fine synthetic resin particles), and there are two types of the one-component developer; a magnetic type and a non-magnetic type. Namely, each toner particle of the magnetic type one-component developer has a resin part and a magnetic fine power part, whereas each particle of the non-magnetic type one-component developer has only a resin part. A developing device using the magnetic type one-component developer is also provided with a magnetic roller, which can be constructed in substantially the same manner as that for the two-component developer. Namely, the magnetic type one-component developer also can be brought to the surface of the photosensitive drum by the rotating magnetic roller as in the developing device using the two-component developer. In a developing device using the non-magnetic type one-component developer, a conductive elastic roller, which may be formed of a conductive foam rubber material, is used as a developing roller. When the conductive elastic roller is rotated within a body of the developer held by a vessel, the toner particles are frictionally entrained to be brought to the surface of the photosensitive drum.
In the developing device using the one-component developer, it is always necessary to bring the toner on the drum to a uniform thickness before an even development of the latent image can be obtained. Namely, a uniform layer of the toner must be formed around the developing roller. To this end, the developing device is provided with a blade member engaged with the surface of the developing roller, to uniformly regulate a thickness of the toner layer formed therearound. The blade member also serves to electrically charge the toner particles by a triboelectrification therebetween. In this case, a material of the blade member is selected such that the toner is charged with a desired polarity. Nevertheless, a charging characteristic of the one-component developer is also affected by a temperature and air moisture content. Generally, the one-component developer is liable to have a low electric charge under the triboelectrication with the blade member, and thus an electrophotographic fog may appear even under normal temperature and normal air moisture content.
The conventional electrophotographic recording apparatus also involves a problem to be solved in the toner image transferring process. The electric discharger used in this process gives the sheet or paper an electric charge having a polarity opposite to that of the developed toner image, whereby the toner image is electrostatically transferred from the photosensitive drum to the paper. A quality of the transferred toner image, and therefore the recorded toner image, depends upon a toner transfer efficiency, and this toner transfer efficiency is also governed by a temperature and air moisture content. Note, the toner transfer efficiency is defined as a ratio of an amount of the transferred toner to a total amount of the toner held by the drum. As the temperature and air moisture content is higher, the toner transfer efficiency is reduced so that a density of the transferred toner image, and therefore the recorded toner image, is lowered.
Furthermore, the electric discharger used in the toner transferred process has an inherent defect in that ozone is produced during the energizing thereof. Not only is ozone injurious to the health, but also it causes a premature deterioration of the photosensitive drum and other parts of the printer. Also, the use of the electric dischargers results in an increase in the production cost of the recording apparatus, because it must be provided with a high voltage electric power source for the electric discharger and an ozone filter for preventing an ozone leakage. Of course, this is also true for the electric discharger used to produce an electrically charged area on the photosensitive drum.